When detecting fluorophore labelled substances in an electrophoretic separation medium, e.g. in connection with DNA sequencing, it is known that upon excitation, at some stage, the signal generated by a detector in response to the detected intensity of the fluorescence emitted by the substances, may fade out, e.g. due to the fact that there is too little fluorophore labelled DNA in the sample. There may also be other reasons for the signal to fade out or be too low to be detected normally.
It is also known that in some applications, e.g. fragment analysis, the opposite situation may arise, namely that the intensity of the fluorescence will be too high for the detector to handle, i.e. the detector will become saturated.
In both these cases, information about the separated substances may be lost. This can e.g. lead to that it will be impossible to determine the DNA sequence.
Proteins and carbohydrates are two other examples of substances that may be labelled with fluorophores and separated in an electrophoretic separation medium.
Patent abstracts of Japan, abstract of JP-A-7-151687, describes the prevention of reading errors of a basic sequence in a fluorescence detection type electrophoretic apparatus by monitoring the intensity of the excitation light. An alarm is generated when the detection signal from a photodetector is smaller than a predetermined reference value because of an abnormal intensity of the emission light. In another prior art device a first image of the fluorescence emitted by said fluorophore labelled substances upon illumination is acquired using a photo-multiplier tube during a first scan at a first voltage setting (and hence gain) and then a second image of the same substances is acquired by a second scan at a different PMT voltage and gain. For example, a first scan could be performed at a high gain setting of 1000 Volts in order to capture an image showing samples emitting a small amount of fluorescence. As samples emitting a high amount of fluorescence will saturate the signal at this gain and it is not possible to quantify the signals if they are saturated then a second scan at a lower voltage, e.g. 400V, which gives a lower gain, and hence gives an unsaturated signal, is necessary. This increases the dynamic range of the sensor but doubles the scanning time.